Stochastic collapse

Miranda, Tays; Frion, Emmanuel; Wands, David

doi:10.1088/1475-7516/2020/01/026

Cited by 9 publications

(7 citation statements)

References 79 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Let us also note that the stochastic formalism was already employed in the context of contracting cosmologies in Ref. [28], where the case of a contracting phase driven by a scalar field with an exponential potential was studied. In such systems, phase space possesses classical unstable fixed points.…”

Section: Resultsmentioning

confidence: 99%

Unavoidable shear from quantum fluctuations in contracting cosmologies

Grain

Vennin

2021

Eur. Phys. J. C

View full text Add to dashboard Cite

Contracting cosmologies are known to be flawed with a shear instability, where the contribution from the anisotropic stress to the overall energy density grows as $$a^{-6}$$ a - 6 , with a the scale factor. Classically, whether or not this contribution becomes important before the bounce depends on its initial value, which can always be sufficiently fine tuned to make it irrelevant. However, vacuum quantum fluctuations inevitably provide a non-vanishing source of anisotropic stress. In this work, we compute the minimum amount of shear that is obtained if one assumes that it vanishes initially, but lets quantum fluctuations build it up. In practice, we consider a massless test scalar field, and describe its quantum fluctuations by means of the stochastic “inflation” (though here applied to a contracting phase) formalism. We find that, if the equation-of-state parameter of the contraction satisfies $$w>-1/9$$ w > - 1 / 9 , regardless of when the contracting phase is initiated, the time at which the shear becomes sizeable is always when the Hubble scale approaches the Planck mass (which is also where the bounce is expected to take place). However, if $$w<-1/9$$ w < - 1 / 9 , the shear backreaction becomes important much earlier, at a point that depends on the overall amount of contraction.

show abstract

Section: Resultsmentioning

confidence: 99%

Unavoidable shear from quantum fluctuations in contracting cosmologies

Grain

Vennin

2021

Eur. Phys. J. C

View full text Add to dashboard Cite

show abstract

“…Let us also note that the stochastic formalism was already employed in the context of contracting cosmologies in Ref. [26], where the case of a contracting phase driven by a scalar field with an exponential potential was studied. In such systems, phase space possesses classical unstable fixed points.…”

Section: Discussionmentioning

confidence: 99%

Unavoidable shear from quantum fluctuations in contracting cosmologies

Grain,

Vennin

2020

Preprint

View full text Add to dashboard Cite

Contracting cosmologies are known to be flawed with a shear instability, where the contribution from the anisotropic stress to the overall energy density grows as a −6 , with a the scale factor. Classically, whether or not this contribution becomes important before the bounce depends on its initial value, which can always be sufficiently fine tuned to make it irrelevant. However, vacuum quantum fluctuations inevitably provide a non-vanishing source of anisotropic stress. In this work, we compute the minimum amount of shear that is obtained if one assumes that it vanishes initially, but lets quantum fluctuations build it up. In practice, we consider a massless test scalar field, and describe its quantum fluctuations by means of the stochastic "inflation" (though here applied to a contracting phase) formalism. We find that, if the equation-of-state parameter of the contraction satisfies w > −1/9, regardless of when the contracting phase is initiated, the time at which the shear becomes sizeable is always when the Hubble scale approaches the Planck mass (which is also where the bounce is expected to take place). However, if w < −1/9, the shear backreaction becomes important much earlier, at a point that depends on the overall amount of contraction.

show abstract

“…Our goal is also to derive the tools required to perform such a transformation on generic grounds, in the full phase space of the separateuniverse system. As mentioned above, another situation where a dynamical attractor is not always available is the case of slowly contracting cosmologies, so the present work can be seen as a prerequisite for the development of a "stochastic-contraction" formalism [12,13], which will be the topic of future works.…”

Section: Introductionmentioning

confidence: 96%

“…Refs. [12,13]). In particular, bouncing cosmologies, where the classical expansion is preceded by a contracting phase and a regular bounce, are typical examples of alternatives to inflation [14,15] arising in various contexts such as string theory or loop quantum gravity, see e.g.…”

Section: Introductionmentioning

confidence: 99%

Hamiltonian formalism for cosmological perturbations: the separate-universe approach

Artigas,

Grain,

Vennin

2021

Preprint

View full text Add to dashboard Cite

The separate-universe approach provides an effective description of cosmological perturbations at large scales, where the universe can be described by an ensemble of independent, locally homogeneous and isotropic patches. By reducing the phase space to homogeneous and isotropic degrees of freedom, it greatly simplifies the analysis of large-scale fluctuations. It is also a prerequisite for the stochastic-inflation formalism. In this work, we formulate the separate-universe approach in the Hamiltonian formalism, which allows us to analyse the full phase-space structure of the perturbations. Such a phase-space description is indeed required in dynamical regimes which do not benefit from a background attractor, as well as to investigate quantum properties of cosmological perturbations. We find that the separate-universe approach always succeeds in reproducing the same phase-space dynamics for homogeneous and isotropic degrees of freedom as the full cosmological perturbation theory, provided that the wavelength of the modes under consideration are larger than some lower bound that we derive. We also compare the separate-universe approach and cosmological perturbation theory at the level of the gauge-matching procedure, where the agreement is not always guaranteed and requires specific matching prescriptions that we present.

show abstract

Stochastic collapse

Cited by 9 publications

References 79 publications

Unavoidable shear from quantum fluctuations in contracting cosmologies

Unavoidable shear from quantum fluctuations in contracting cosmologies

Unavoidable shear from quantum fluctuations in contracting cosmologies

Hamiltonian formalism for cosmological perturbations: the separate-universe approach

Contact Info

Product

Resources

About